Thermoplastic norbornene polymers obtained by subjecting a norbornene monomer to ring-opening polymerization or addition polymerization in the presence of a metathesis catalyst system and optionally hydrogenating a polymer formed after the polymerization are excellent in various properties such as transparency, insulating properties, low dielectric properties, heat resistance, moisture resistance and mechanical strength. Therefore, investigations are being made as to the applications of such norbornene polymers to various uses such as optical materials, medical materials and semiconductor materials. Among these uses, for example, medical bottles, syringes, vials, housings, casings, insulating materials, etc. are particularly required to improve their strength properties and resistance to discoloration.
As processes for producing norbornene polymers, there have heretofore been proposed, for example, the following various processes:
(1) In Japanese Patent Publication No. 4528/1989, there is proposed a process in which dicyclopentadiene is subjected to ring-opening polymerization using a metathesis catalyst system composed of WCl.sub.6 /Bu.sub.4 Sn (Group IVB)/Et.sub.2 O. According to this process, however, the molecular weight distribution (Mw/Mn) of the resulting polymer becomes wide, and so its processability is lowered if its molecular weight is made high for the purpose of improving its strength properties.
(2) In Japanese Patent Publication No. 43412/1983, there is proposed a process in which dicyclopentadiene is subjected to ring-opening polymerization using a metathesis catalyst system composed of TiCl.sub.6 /Et.sub.3 Al/Et.sub.3 N. According to this process, however, the molecular weight distribution of the resulting polymer cannot be sufficiently narrowed, and so it is difficult to balance the strength properties and processability of the polymer with each other at a high level. In addition, this process also involves a drawback that since the catalyst is used in a great amount, a great amount of a metallic catalyst residue is left in the polymer formed, and so the polymer undergoes discoloration.
(3) In Japanese Patent Application Laid-Open No. 197460/1989, there is proposed a process in which a norbornene monomer is subjected to ring-opening polymerization using a metathesis catalyst system composed of WCl.sub.6 /i-Bu.sub.3 Al/paraldehyde. According to this process, however, gelation tends to occur in the course of the reaction, and so it is difficult to produce a norbornene polymer having a high molecular weight and a sufficiently narrow molecular weight distribution.
(4) Japanese Patent Application Laid-Open Nos. 179575/1995, 188396/1995 and 206999/1995 each disclose a process for producing a norbornene polymer having a ratio (Mw/Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 1.36 or lower by a living metathesis polymerization method. However, this process involves a drawback that since a catalyst is used in a great amount due to living polymerization, a great amount of a metallic catalyst residue is left in the resulting norbornene polymer. In addition, the processability of such a polymer is insufficient.